The process of antigenic variation in African trypanosomes allows the survival of the parasite by constantly switching the variant surface glycoprotein (VSG) expressed in their surface. There are believed to be several hundred copies of these silent VSG genes in the parasite's genome and they are expressed differentially. The majority of these genes are not capable of being transcribed in situ and must therefore be expressed from specialised transcriptional units known as bloodstream expression sites (BESs). Only one such site is active at any one time, ensuring that a single VSG is expressed in the trypanosome's surface coat. Switching the expressed VSG involves replacing the VSG in the active BES, or activating a new BES in conjunction with silencing the previously active. Differential expression of variant surface glycoprotein (VSG) genes, has a strong association with telomeres. All BESs are telomeric and differential activation involves recombination into the telomeric environment or silencing/activation of subtelomeric promoters. A number of pathogen contingency gene systems associated with immune evasion involve telomeric loci, which has prompted speculation that chromosome ends provide conditions conducive for the operation of rapid gene switching mechanisms. Ku is a protein associated with yeast telomeres that is directly involved in DNA recombination and gene silencing. The main aim of this thesis was to test the hypothesis that Ku in trypanosomes is centrally involved in differential VSG expression. In order to compare trypanosome Ku homologues with those from other organisms, it was necessary to compile homology alignments with other Ku homologues using Clustal W analysis. Subsequent experiments looked at the fate of exogenously introduced restriction enzyme target sites after transient transformation with cassettes encoding the restriction enzyme. A final analysis looked for the presence of NHEJ in homologous recombination- deficient trypanosomes. Disrupting this element of DNA repair would hopefully lead to other forms of repair becoming detectable, and even up-regulated. Rad51, in yeast a member of the Rad52 epistasis group (integral in yeast homologous recombination), had previously been demonstrated to be involved in DNA repair in trypanosomes (McCulloch & Barry, 1999). rad51 mutants were electroporated with cassettes containing noncompatible ends that would prevent their integration into the endogenous genome via conventional homologous recombination. This cassette also contained promoter DNA sequence to allow selection in the event of integration into non-transcribed regions of the genome. Study of the junctions encompassing the integration sites of the cassette allowed investigation into how the cassettes were integrated, and revealed to us the extent of the sequence homology required to catalyse integration. The method of repair detection observed indicated that classical homologous recombination is not the only pathway utilised by African trypanosomes to metabolise DNA double-strand breaks.

In recent times a number of workers have pointed out the importance of microcrack formation to the machining process and to surface integrity requirements of machined components. Microcracks have been identified within the shear zone and their presence used to explain some aspects of the chip formation process. It is clear that microcracks represent a major feature in the shear zone in machining, particularily when the workpiece is a two-phase material. The presence of microcracks in the machined surface was reported some fifteen years ago and confirmed recently. A preliminary study carried out on a number of materials machined under various cutting conditions has indicated that microcracks are produced in machined surfaces as a result of machining. However, to the present time there has been no systematic study of the formation of microcracks in the machined surface, and their extent, dimensions, and the conditions under which they are produced are completely unknown. In so far as microcracks may affect the fatigue, corrosion and other properties of the machined surface, it is important that a method of quantifying microcrack formation should be found. A feasibility study has been carried out which has led to the use of the transmission electron microscope as a tool to quantify microcrack formation in the machined surface. A technique based on transmission electron microscopy is described in the present thesis along with its application. Two new parameters, the microcrack area ratio "Ma" and the microcrack density "Md" are introduced and defined. The extent of microcrack formation in the machined surface is studied as a result of varying: (i)- cutting speed, (ii)- undeformed chip thickness, (iii)- rake angle, (iv)- workpiece hardness. The results are discussed and conclusions drawn. Finally, a number of suggestions for future work are formulated which mark the start of a whole new area of research in surface microcracking.

In this research project the author introduces the use of geometrical modelling techniques alongside geostatistical methods to model a stratigraphic orebody and to present a graphics display system developed as a fIrst step towards a general integrated system for computer aided design and planning in mining. Geometrical modelling techniques and geostatistical methods are combined to carry out the process of modelling a stratigraphic orebody. From a mining point of view, there are two main features of interest in a stratigraphic ore-body: a-The modelling of the geometry of the orebody. b- The modelling (estimation) of the physical properties (grades, etc ... ) of the orebody. The first feature is the subject of this research project. Modelling methods and techniques developed elsewhere and for different applications, such as Computer Aided Design, have been applied successfully to model the geometry of stratigraphic orebodies. The modelling process consists of the applications of surface modelling techniques to represent the hangingwall and the footwall of the stratigraphic orebody and thereby to produce the space where the physical properties are geostatistically to be estimated. The graphics display system is presented to highlight the use of computer graphics techniques to communicate graphically all sorts of information concerning the modelling of stratigraphic orebodies and also to display the end product of the modelling process, such as cross-sections, plane-sections, wireframe and solid models of the orebody. The graphics system itself is part of a computer based system for mine design and planning similar to computer aided design systems used mainly in the manufacturing industry. The presentation of the research project in this thesis started by the review of the literature of some existing ore reserves estimation methods in the mineral industry, particularly geostatistical methods. Then an overview and the scope of this research project have been given. The second chapter describes the type of data which could be encountered while building a geometrical model of a stratigraphic orebody and a description of data from a nickel vein deposit used as a case study for this research project. The accumulations have been estimated geostatistically subject to geometrical control. The geometrical control concept and surface modelling techniques are presented in chapter three together with the numerical application of modelling a nickel vein deposit using two different surface modelling techniques. Chapter four describes the graphics display system developed to display several geometric features of stratigraphic orebodies in two and three dimensions. The summary of this research project and some concluding remarks are given in chapter five.

Le recyclage des élastomères est devenu important pour des raisons environnementales et financières. Une forme de recyclage consiste à dévulcaniser l’élastomère afin de le réintroduire comme matière première. Il s'agit d'un processus délicat car l'objectif est de rompre le réseau polymère en brisant les ponts soufre sans endommager les chaînes polymères. Ce travail vise à étudier un procédé de dévulcanisation appelé "High Shear Mixing" (HSM). Une machine a été conçue, construite et instrumentée. Dans le procédé HSM, l’élastomère est cisaillé entre deux cônes ayant des géométries spécifiques. Pendant ce cisaillement, la matière est auto-chauffée, un système de refroidissement limite l’augmentation de température. Les objectifs de l’étude étaient de mieux comprendre le procédé et les processus physiques, de déterminer un paramètre procédé pour suivre l’évolution de la matière au cours du traitement et d’optimiser le procédé. Notre analyse a mis en évidence 2 paramètres: la température de l’élastomère et l'énergie consommée au cours du procédé. Cette énergie peut être corrélée au degré de dévulcanisation qui est mesuré par des analyses physico-chimiques. L’activation des surfaces d’élastomère créées pendant le traitement a été mis en évidence. Un modèle physique de l'évolution du réseau polymère est proposé. Pour valider l'efficacité du recyclage, l'effet de l'ajout de différentes fractions de la matière traitée dans la même formulation de matière première a été testé. Une méthode Taguchi a été utilisée pour concevoir les formulations et les facteurs HSM à tester. Les mélanges reformulés ont été vulcanisés et leurs propriétés ont été analysées. / The recycling of rubber in the industry has become highly important due to environmental and financial reasons. A recycling approach is to devulcanize the rubber in order to reintroduce it as a raw material. This is a challenging process since the objective is to rupture the rubber network by breaking sulfur bridges without damaging the polymer chains. This work aims to study a devulcanization process known as “High Shear Mixing” (HSM). A machine was designed, built and instrumented with the purpose of studying the different phenomena occurring during the devulcanization process. During the treatment, the rubber is sheared between two cones with special geometries. During this shearing the treated rubber is self-heated, a cooling system prevents the rubber degradation due to excessive heat. The objectives were to better understand the process and the physics behind, to determine a process parameter to follow the rubber evolution and to optimize the treatment. Our analysis highlighted 2 parameters: the rubber temperature and the specific mechanical energy consumed during the process. This energy can be correlated to the degree of devulcanization of the rubber which is measured by means of physicochemical analyses. An optimal state of surface activation on the treated rubber was also described. A physical model of the rubber network evolution along the HSM treatment is proposed. To validate the recycling efficiency, the effect of adding different fractions of treated rubber in the same raw material formulation was tested. A Taguchi method was used to design formulations and HSM factors to be tested. Reformulated mixtures were vulcanized and rubber properties were analyzed.

An attempt to minimize interference drag in a wing-fuselage junction by means of inserting a fillet is presented in this thesis. The case of a low-wing com- mercial transport aicraft at cruise conditions is examined. Due to the highly three dimensional behaviour of the flow field around the junction, a thin-layer Navier-Stokes code was implemented to estimate the drag forces at the junc- tion. Carefully selected design variable combinations based on-the theory of Design of Experiments constituted the initial group of feasible cases for which the flow solver had to be run. The drag values of these feasible cases were then used to create a second order response surface which could predict with rea- sonable accuracy the interference drag given the value of the design variables within the feasible region. A further optimization isolated the minimum in- terference drag combination of design variable values within the design space. The minimurn interference drag combination of design variable values was eval- uated numerically by the flow solver. The prediction of the response surface and the numerical value obtained by the flow solver for the interference drag of the optimal wing-fuselage combination differed by less than five percent. To demonstrate the ability of the method to be used in an interdisciplinary analysis and optimization program, a landing gear design module is included which provides volume constraints on the fillet geometry during the fillet sur- face definition phase. The Navier Stokes flow analyses were performed on the Cranfield Cray su- percomputer. Each analysis required between eight to twelve CPU hours, and the total CPU time required for the optimization of the six variable model described in the thesis required thirty Navier Stokes runs implementing the Design of Experimens and Surface Response Methodology implementation. For comparison, a typical optimization implementing a classical conjugate di- rections optimizer with no derivative information available would probably require more than forty iterations. Both the optimization and the flow solver results are discussed and some recommendations for improving the efficiency of the code and for further ap- plications of the method are given.

A Seifert surface for a knot in ℝ³ is a compact orientable surface whose boundary is the knot. Seifert surfaces are not unique. In 1934 Herbert Seifert provided a construction of such a surface known as the Seifert Algorithm, using the combinatorics of a projection of the knot onto a plane. This thesis presents another construction of a Seifert surface, using differential geometry and a projection of the knot onto a sphere. Given a knot K : S¹⊂ R³, we construct canonical maps F : ΛdiffS² → ℝ=4πZ and G : ℝ³ - K(S¹) → ΛdiffS² where ΛdiffS² is the space of smooth loops in S². The composite FG : ℝ³ - K(S¹) → ℝ=4πZ is a smooth map defined for each u∈2 ℝ³ - K(S¹) by integration of a 2- form over an extension D² → S² of G(u) : S1 → S². The composite FG is a surjection which is a canonical representative of the generator 1∈H¹(ℝ³- K(S¹)) = Z. FG can be defined geometrically using the solid angle. Given u ∈ ℝ³ - K(S¹), choose a Seifert surface Σu for K with u ∉ Σu. It is shown that FG(u) is equal to the signed area of the shadow of Σu on the unit sphere centred at u. With this, FG(u) can be written as a line integral over the knot. By Sard's Theorem, FG has a regular value t ∈ ℝ=4πZ. The behaviour of FG near the knot is investigated in order to show that FG is a locally trivial fibration near the knot, using detailed differential analysis. Our main result is that (FG)-¹(t)⊂ ℝ³ can be closed to a Seifert surface by adding the knot.

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The technique of recursive subdivision can be visualised, loosely, as successively chopping off the corners of a polyhedron to make it less pointed. If the polyhedron is represented as a mesh of points connected by edges, repeated application of the subdivision results in progressively finer meshes tending in the limit to a surface. The subdivision is determined by the weightings given to the respective points and their neighbours.

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No description available.